The invention relates to an acoustical method and apparatus for distance measurement.
A method for distance measurement that a developer employs in each distance measurement application could be based on the Pulse Transit Time (PTT) principal, as disclosed in U.S. Pat. Nos. 5,877,997, 5,822,275 and 5,793,704, or could be a continuous FMCW or Phase Shift or Amplitude Change monitoring methods or their multiple variations, as disclosed in U.S. Pat. Nos. 6,040,898 and 5,867,125. Regardless of the particular acoustical method for distance measurement, the transience of the speed of sound is always present and it always negatively affects the accuracy of measurement. For example, considering air, to be a transmitting medium in the acoustical measuring system, it is a known fact that the speed of sound depends on air pressure, air temperature and humidity along the path of the sound propagation, and the air along this path is not homogeneous, Meyers et al. Therefore, it could be a very complex task to equip an acoustical distance-measuring device with various sensors required for compensation of the mentioned above variables. Naturally, the need in effective compensation of the speed of sound changes during the acoustical distance measurement attracts attention of the acoustic measuring systems designers.
One solution was the direct measurement of the speed of sound and the consequent use of the result of this measurement in the distance-computing algorithm or the structure of the distance measuring analog and/or digital subsystem. Such approach is described in the U.S. Pat. No. 5,867,125 to Cliff et al. The Cliff patent recommended that if both, a transmitter and receiver are stationary, the system is capable of measuring changes in the propagation velocity of the transmission medium. This measurement could be used as a feedback to a system, allowing compensation for the changed propagation velocity. For example, this measurement could be used to adjust the oscillator of an ultrasonic range finding system so that the wavelength of the ultrasound does not vary with atmospheric changes.xe2x80x9d
Another solution suggested utilization of the natural properties of the measuring system itself for compensation of the speed of sound fluctuations. In such a case, the speed of sound compensating feedback could be found with the aid of selection of the specific fiducial points on the characteristic of the variable carrying information on the sought distance. An example of this approach is described in the U.S. Pat. No. 5,793,704, issued to Freger. In this work, the method of measurement was based on the Pulse Transit Time principle that requires sending pulse trains of acoustic energy toward a target, obtaining pulse trains reflected by the target, and calculating the sought distance as a function of the pulse travel time and the speed of sound. The author had discovered that any sensor used for the detection of the reflected pulses responded nonlinearly to the onset of acoustical energy. xe2x80x9cThese sensors are characterized by a response time that is shorter for high-energy signals than for low-energy signals.xe2x80x9d Furthermore, the patent stated that the energy level of the echo pulses propagating through the air tends to change inversely to the velocity of the sound propagation. Therefore, the author concluded that a range measurement based on selecting the maximum of the envelope of the received echo pulses is relatively immune to variations in the speed of sound.
The analysis of the former approach to compensation of the speed of sound variations during the acoustical distance measurement shows that this solution has a limited use due to the requirement that the only variable that may vary is the speed of sound; all other method""s relating variables must be stationary in order to correctly measure the speed of sound changes. This condition leads a designer to the creation of a separate speed of sound measuring subsystem where the distance between the transducer and the target is known and stationary or the receiver serves as the target in a stationary arrangement with the known distance between the transmitter and the receiver, as well as the ambient air temperature, humidity, and the ambient air composition and dynamics. In some cases, the additional difficulty in the utilization of this method is conditioned by the inability for the distance measuring subsystem and the speed of sound measuring subsystem to operate in the same spatial field bringing inaccuracies to the outcome of the distance measurement.
The later solution to the reduction of the harmful effect of the speed of sound variations on the distance measurement does not provide for the total exclusion of the speed of sound from the measuring system equation. In the measuring systems designed on this principle, there are certain residual inaccuracies in the distance measurement that are attributable to the transience of the speed of sound.
Regardless of the acoustical method for distance measurement, some minimization of the speed of sound-caused disturbance (fast changes) could be achieved with the aid of signal filtering in echo processing. However, such improvement is obtainable at the expense of slowing the measuring system operating speed. Therefore, there is a need for an improved method and apparatus for measuring distances.
One object of the present invention is development of a method for distance measurement providing for the total exclusion of the speed of sound from the measuring system governing equation.
Another object of the present invention is to design a distance measuring apparatus implementing the method of measurement completely excluding the speed of sound from the factors affecting the accuracy of measurement.
According to the present invention, a method for distance measurement improves accuracy, resolution and operating speed of the acoustical distance measurement by means of complete exclusion of the speed of sound propagation in the measuring system""s sound conducting medium from set of variables affecting the measurement. The method includes the steps of establishing two spatially different points from which a distance is measured or monitored by acoustical measuring devices between each said point and a center of a reflecting area on a target object such that a substantially perpendicular line could be drawn through said first point and said center of said reflecting area on said target, and an angled line could be drawn through said second point and said center of said reflecting area on said target, thereby these two lines and a line drawn through said two points define a right triangle denoted xcex94ABC with its hypotenuse denoted CB between said second point and said center of said reflecting area of said target; measuring an approximate length of a vertical leg AB of said triangle; measuring an approximate length of said hypotenuse CB of said triangle; and obtaining a distance defined, between the said first point and said center of said reflecting area on said target through the application of the functional relationship between said distance and a ratio between the approximate length of said vertical leg AB of said triangle and the approximate length of said hypotenuse CB of said triangle, whereby said distance measurement is invariant to changes in the speed of sound propagation through the medium.
The implementation of the above identified method steps enables an acoustical measuring system to measure distance independent of the varying speed of sound propagation through the medium.
One embodiment of the present invention includes an installation of two assemblies of acoustical transmitting and receiving devices such that a first assembly sends and receives acoustic energy along a perpendicular axis to a reflecting area disposed on a target, and a second assembly sends and receives acoustical energy along an angled axis to the same reflecting area on the target. The direction of the sound propagation between the two assemblies and the target makes a substantially right triangle with its hypotenuse between an emitter of the second assembly""s and an instant center of the target""s reflecting area.
The method allows several embodiments. Particularly, for a PTT-based approach to distance measurement, the formula for the sought distance computation transforms into the following expression: h=wxc2x7tg[arcsin(T1/T2)], wherein T1 denotes the time that passes while the acoustic pulse train travels from the first assembly""s emitter toward the target and back to the first assembly""s receiver; T2 denotes the time that passes while the acoustic pulse train travels from the second assembly""s emitter toward the target and back to the second assembly""s receiver. One possible positioning of the transmitter and the receiver of the second assembly is that the receiver and the transmitter are positioned at opposite sides from the first assembly transducer; the second assembly emitter and receiver and the first assembly transducer are located on the same plane or parallel planes, that are parallel to the target""s reflecting area too. The embodiment also includes an Electronic Processing and Distance Computing Unit, (xe2x80x9cUnitxe2x80x9d hereafter). The output of the first assembly transducer is connected to the first input of the Unit. The output of the second assembly receiver is connected to the second input of the Unit, therefore, facilitating the ability of the electronic processing unit to process echo signals and to compute the sought distance between the first assembly transducer and the target and to display and/or to deliver the result of measurement to any accepting device or a user.
The foregoing and other advantages of the present invention become more apparent in light of the following detailed description of the exemplary embodiments thereof, as illustrated in the accompanying drawings.